1. Field of the Invention
The present invention relates in general to devices for measuring a distance and more particularly to distance measuring devices of a laser beam using type which, for measuring the distance to an object, emits a going laser beam toward the object and detects a returning laser beam from the object. (The returning laser beam is the going laser beam which has been reflected by the object.) In practical use, the period required for the laser beam to make the round trip is detected. The distance to the object is equal to the one-half of the time elapsed multiplied by the velocity of the laser beam.
2. Description of Related Art
FIG. 4 shows schematically an electric circuit installed in a conventional distance measuring device of a laser beam using type. Designated by numeral 101 is a laser output circuit 101 which has a laser emitting section 102 connected thereto. Upon energizing the laser output circuit 101, a going laser beam "LT" is emitted from the laser emitting section 102 toward a remote object (not shown) and at the same time a part "L1" of the going laser beam "LT" is sensed by a going laser beam sensor 103. In fact, between the laser emitting section 102 and the going laser beam sensor 103, there is arranged a reference optical track 108 through which the part "L1" of the going laser beam "LT" travels before being led into the going laser beam sensor 103. Thus, the part "L1" of the laser beam "LT" is referred to as a reference laser beam. A returning laser beam "LR" which has been reflected by the object is sensed by a returning laser beam sensor 104. Each laser beam sensor 103 or 104 converts the light energy of the sensed laser beam to corresponding electric wave signal "T1" or "R" (see FIGS. 5A and 5B). The two laser beam sensors 103 and 104 are connected to a wave processing circuit 107 through respective amplifying circuits 105 and 106.
As is seen from FIGS. 5A-5C, in the wave processing circuit 107, the wave signal "R" of the returning laser beam "LR" and the wave signal "T1" of the reference laser beam "L1" (viz., the wave signal of the going laser beam "LT") are processed to produce a series of pulse signal "PS" whose pulse width "t" corresponds to the phase difference between the returning laser beam "LR" and the reference laser beam "L1" (viz., the going laser beam "LT"). More specifically, the pulse width "t" represents the period required for the going laser beam to make the round trip.
As is seen from FIG. 4, the wave processing circuit 107 outputs a distance representing voltage signal "RD" which is based on the pulse width "t".
However, due to its inherent construction, the above-mentioned conventional distance measuring device tends to lower its performance particularly when used in a high temperature place. That is, when heated due to inevitable heat generation of the device in use and/or heat possessed by surrounding air, the distance representing voltage signal "RD" issued from the wave processing circuit 107 tends to have a no small error. This is because the signal treatment is influenced by the amplitude of the sensed laser beam and the fluctuation of the threshold of the comparator. In other words, the signal treatment is influenced by the temperature drift of the electric circuit elements of the sensor, amplifier, comparator, etc. As a result of this fact, the wave processing circuit 107 processes an electric wave signal "T2" (see FIG. 5B) which is advanced (or delayed) in phase with respect to the desired (or proper) electric wave signal "T1". Thus, in this case, the pulse signal "PS" has an erroneous pulse width "te" which is greater than the actual pulse width "t", and thus the distance representing voltage signal "RD" from the wave processing circuit 107 has no small error.